Apparatus for carrying out physical and/or chemical processes, more specifically a heat exchanger of the continuous type

ABSTRACT

This invention relates to apparatus for carrying out physical and/or chemical processes, more specifically a heat exchanger (10) of the continuous type, comprising a large number of vertical heat exchanger tubes (18) mounted between a lower chamber (17) and an upper chamber (16) within a jacket (15). Within the tubes (18) small solid particles are kept in a fluidized condition by the fluid medium passing up from the lower chamber (17) through the tubes (18), so as to achieve a good heat transfer, whereas also any deposit will be removed from the inner tube walls. For the downward flow of the granular mass at least one return tube (21) is provided. Downward circulation of the fluid medium through the return tube (21) should be minimal, however, which can be achieved by an increase of the pressure difference between the upper and lower chamber (16; 17), which increase of the pressure difference can be achieved by fitting several distribution plates (24; 42) at a level above the return tube (21) outlet into the lower chamber (17), preferably so that the apertures in the various distribution plates (24; 42) of the multiple design are not vertically in line. The second distribution plate (42) also serves to minimize excessive wear of the material of the jacket (15) due to the eddying motion of the granules.

The invention relates to apparatus for carrying out physical and/orchemical processes, more specifically a heat exchanger of the continuoustype, comprising a bundle of parallel vertical riser tubes, an upperchamber, a lower chamber, upper and lower tube plates for openconnection of the tube bundle with the upper and lower chambersrespectively, a granular mass that can be kept in a fluidised condition,at least in the riser tubes, by a fluid medium flowing during operationupwardly through the lower chamber, the riser tubes and the upperchamber, a distribution plate for the granular mass in the lowerchamber, and at least one return tube opening below the distributionplate for return of an overflow of granular mass above the upper tubeplate from the upper chamber to the lower chamber, wherein each risertube is provided with an inflow element extending in the lower chamber,from the lower tube plate to a level above the distribution platethrough which the return tube or return tubes projects or project, andwherein the lower chamber is provided with a device which preventsgranules from reaching the lower chamber inlet for the fluid mediumduring standstill.

An apparatus of this type is known from an article by D. G. Klaren in"Fouling Prevention Research Digest", Vol. 5, No. 1 pp. III-XVII (March1983). The known heat exchanger that because of the presence of thefluidised bed has a high efficiency as a result of breakage of the fluidfilm along the inner surface of the riser tubes is primarily ofinterest, if, from the fluid medium on the inner surface of the risertubes, a layer of material can be deposited that impedes heat transferthrough the riser tube walls. This impeding layer is not deposited inthe known apparatus due to the abrasive action of the granules.Therefore the known apparatus is ideal for application in e.g. the foodindustry.

On the other hand the presence of the granular mass brings itsdisadvantages. One disadvantage of the known apparatus is that it isnever completely certain that the downward flow of the granular masswill occur only in the return tube or the return tubes that are intendedfor the purpose. To achieve a correct design and to ensure an evendistribution of both the fluid medium and the granular mass across allriser tubes it is of utmost importance that only tubes that are intendedfor the purpose serve as return tubes. The situation where one or moreriser tubes will still start to function as return tubes can occur inparticular if insufficient care is exercised at start-up of the heatexchanger. It is then possible that the fluidised granules insideseveral riser tubes reach the top ends thereof earlier than the granulesin other riser tubes, which causes the granules issuing from the formerriser tubes to flow through the upper chamber into the adjoining risertubes that are not yet completely filled with fluidised granules. Theresult is disturbance of the pressure balance between the various tubescausing a downward flow of fluid medium and granules into one or severaltubes that were not originally intended for such a downward flow.

The invention eliminates this first objection by means of a constrictionat the ends of the riser tubes that open into the upper chamber.

In this constriction the velocity of the fluid medium is increased tosuch an extent that the granules cannot drop from the upper chamber backinto a riser tube. With the constriction correctly dimensioned, even ahigh irregular distribution of the granular mass across the riser tubeswith startup of the heat exchanger cannot cause one or several risertubes to start acting as return tubes.

An advantageous construction of the apparatus is characterised by anupper pipe plate provided with apertures for the riser tibes havingsmaller cross-sections than the riser tubes themselves.

The granular mass circulates in the upward direction through the risertubes and in the downward direction through the return tube. During thiscirculation, particularly in operation, granules are in eddying motionalso in the lower chamber that apparently are the source of very severewear of the heat exchanger jacket, especially of the material at thejunction of the bottom cover and the cylindrical wall which forms partof the shell.

The second disadvantage is eliminated according to the invention bymeans of a second distribution plate arranged in the lower chamber at alevel below the outlet therein of the return tube.

In this proposed construction, in contrast with the known apparatus, thedevice that impedes the granules from reaching, at standstill, the inletfor the fluid medium in the lower chamber cannot serve as a shutoffvalve against the lower edge of the return tube. Consequently under allconditions a downward flow of the fluid medium will occur inside thereturn tube which, in the construction proposed, can amount to approx.25 percent of the fluid medium mass flow supplied to the heat exchangerinlet. As the result the average logarithmic temperature differenceinside the riser tubes will be less favourable which requires that, toachieve a desired efficiency, a larger heated surface must be installed.

The last requirement can be obviated to a certain degree according tothe invention by constructing the distribution plate at a level abovethe return tube outlet into the lower chamber in multiple; preferably sothat the apertures in the various distribution plates of the multipleconstruction are not vertically in line.

It appears that the downward flow of the fluid medium through the returntube or the return tubes can thus be limited to about 15 percent of themass flow through the heat exchanger inlet.

The absence of any moving parts in the lower chamber, i.e. of theprovision as a valve of the device that prevents granules from reaching,at standstill, the fluid medium inlet into the lower chamber, and thereduced wear of lower chamber parts offset the disadvantage of fluidmedium circulation through the return tube.

To stabilise the fluidised bed in a riser tube it has been suggested toprovide the inflow pipe element with a lateral bore at a level where,during operation, no granules are present in the lower chamber, i.e.where only the fluid medium is present. If strongly contaminating fluidmedia are used, e.g. fluid media that carry solids, the lateral bore mayget clogged causing the fluidised bed in the riser tube to sag andfinally causing the unintended functioning as a return tube of the risertube, which must be defined as objectionable.

In a method of operation of apparatus of the type described at theoutset, this last disadvantage is avoided by temporarily reducing thepressure of the fluid medium at the top of the lower chamber.

According to the invention, the apparatus of the type described at theoutset is thereby characterised by lateral bores provided in the inflowpipe elements. The top of the lower chamber above the level of thelateral bores is fitted with an outlet with shutoff valve.

In particular the apparatus according to the invention is characterisedin that the outlet at the top of the lower chamber is directly connectedwith a fluid medium outlet of the upper chamber.

The presence of a return tube or of return tubes has the associateddisadvantage of causing displacement of the fluid medium from the upperchamber to the lower chamber. Although the granular mass flow inside thereturn tube will under some conditions stagnate and as it were clog upthe return tube, in that case the intended average logarithmictemperature difference between the outer and inner surfaces of the risertubes required for proper functioning of the heat exchanger will beadversely affected, unless by chance an operating point is found wherethis is not the case. However, such an operating point will entailundesirable restrictions and limitations of other operating parameters.

To be able at least to reduce this adverse effect the apparatus of thetype described at the outset is characterised by a bypass between theinlet for the fluid medium used in the process in the lower chamber anda point in the upper chamber where the bypass opens into the upperchamber at the level of the inlet opening of the return tube or tubes.

Thus the bypass, which is preferably fitted with an adjustable valve tocontrol the flow through it, will at least partly eliminate thetemperature difference between the upper and the lower chambers.

According to the invention another solution of the same problem isprovided in that a throttle device is located before the inlet openingof the return tube, which gives precedence to passage through the returntube to the fluid medium over that of the granular mass.

If the quantity of granular mass inside the return tube is thus limitedless driving power will be available for circulation, which will causereduced circulation through the return tube of granular mass and fluidmedium.

Fouling can also occur on the outer wall of the riser tubes due to thenature of the second fluid medium used in the heat exchanging process.This particular problem relates to a more general class of heatexchangers, i.e. of the type where the second fluid medium that is usedin the heat exchanging process is a fluid that, like a film, flows downthe vertical riser tubes of apparatus for carrying out physical and/orchemical processes, more in particular a heat exchanger of thecontinuous type, consisting of a bundle of parallel vertical risertubes, an upper chamber, a lower chamber, an upper pipe plate and alower pipe plate for open connection of the pipe bundle with the upperand lower chambers, respectively, all for throughflow of the first fluidmedium and an upper distribution chamber and a lower collecting chamberfor throughflow of the second fluid medium, the upper distributionchamber closing around each riser tube leaving a gap in the bottom ofthe upper distribution chamber around each riser tube and the top of thelower collecting chamber being provided with collecting apertures aroundeach riser tube.

Such apparatus is known from the article by D. G. Klaren in "FoulingPrevention Research Digest", Vol. 5, No. 1, pp. III-XVII (March 1983).

If the temperature and pressure occuring do not require that the fluidthat is the second fluid medium is enclosed by a fixed jacket wall, thetubes can be easily cleaned from the outside, e.g. by washing down thepipe bunldes which may be done during operation. However, if a verystrongly fouling fluid is used as the second fluid medium in the heatexchanging process the bundles must be washed down so frequently thatthis is not practicable. Even if the process temperature and pressure dorequire that the second fluid medium is enclosed by a fixed jacket wall,this method for pipe bundle washing is out of the question. According tothe invention optimal methods for cleaning the outer surfaces of theriser tubes are provided, in that all tubes have been fitted on theoutside with a scraping device in a supporting construction that can bedriven mechanically.

The scraping device of this type cannot reach the gaps in the bottom ofthe upper distribution chamber that may get clogged due to fouling.

To prevent the gaps from thus clogging up, or to at least delay theclogging process, the upper distribution chamber is provided, above itsbottom, with a distribution shell that has apertures for distribution ofthe fluid over the gaps in the bottom.

Thus the gaps in the bottom can be wider without substantiallyincreasing the height of the upper distribution chamber.

The invention in all its aspects will be illustrated below with adescription referring to a plan. In the plan

FIG. 1 shows an embodiment of a first aspect of the invention;

FIGS. 2 and 3 show embodiments of the invention according to a secondaspect;

FIGS. 4 and 5 show embodiments of the invention according to a thirdaspect;

FIG. 6 shows an embodiment of the invention according to a fourthaspect;

FIGS. 7 and 8 show an embodiment of the invention according to a fifthaspect;

FIGS. 9 and 10 show an embodiment of the invention according to a sixthaspect; and

FIGS. 11 and 12 show an embodiment of the invention according to aseventh aspect.

FIG. 1 shows an embodiment of the apparatus according to the firstaspect of the invention. The heat exchanger 10 has, for a first fluidmedium, an inlet 11 and an outlet 12 and, for a second fluid medium, aninlet 13 and an outlet 14. Inside the jacket 15 a lower chamber 17connects to the inlet 11 and an upper chamber 16 connects to the outlet12. A riser tube bundle 18 is arranged between an upper pipe plate 19and a lower pipe plate 20, the riser tube bundle including a return tube21. The riser tubes are provided with inflow pipe elements 22 thatextend from the lower pipe plate 20 downwardly into the lower chamber17. The inflow pipe elements are provided with lateral bores 23. In thelower chamber 17 a distribution plate 24 is located below the inlets ofthe riser tubes and above the outlet of the return tube. The lowerchamber 17 is further provided with a device 25 that prevents thegranules in the lower chamber, the riser tubes, the upper chamber andthe return tube from reaching the inlet 11. The device 25 is mounted ona spring assembly 26 which, at standstill, presses the device 25 againstthe outlet aperture of the return tube. In any case the granules can bebrought into the fluidised bed condition in the riser tubes 18. Thelower end of the return tube 21 will contain a stagnating granular mass27.

According to the invention the upper pipe plate 19 of the heat exchanger10 is provided with throttling apertures 32 via which the riser tubes 18open into the upper chamber 16. For instance the apertures 32 areprovided in a throttle plate 31 constituting, within the assembly, anintegral part of the upper pipe plate 19 and not forming an obstacle forthe return tube 21.

It will be obvious that the throttling apertures 32 in the outlets ofthe riser tubes 18 are effective to prevent granules from dropping intothe riser tubes from the upper pipe plate independently of the valveaction of the device 25.

FIGS. 2 and 3 show embodiments of the invention according to a secondaspect. The design of the apparatus in these embodiments corresponds inmost respects with that shown in FIG. 1. To prevent the material of thejacket, particularly at the junction of the bottom cover 43 to thecylindrical wall 44, from being excessively worn due to the eddyingmotion of the granules constituting the material of the fluidised bedsin the riser tubes, a second distribution plat 42 is provided in thelower chamber 17 at a level below the outlet in the lower chamber 17 ofthe return tube 21. The device that prevents granules from reaching, atstandstill, the inlet 11 in the lower chamber 17 is provided as a simplebell 41 which is mounted fixedly and distributes the fluid medium fromthe projecting inlet 11 laterally through the lower chamber 17.

At standstill of the heat exchanger the majority of the granular masswill be present in the lower chamber, below the first distribution plate24. At start-up the fluid medium flow from the inlet 11 will try tofollow the least obstructed route. Initially the fluid medium will flowmainly through the return tube 21, but as the granular mass is carriedin fluidised state by the fluid medium flow the medium passing throughthe return tube will gradually meet with greater resistance, causing thefluid medium to start rising also in the riser tubes 18 withsimultaneous fluidisation of the granular mass inside the riser tubes. Afurther increase of the fluid medium mass flow will cause the fluidisedbeds inside the riser tubes 18 to rise faster than the fluidised bedinside the return tube 21. This is due to the lateral bores 23 in theinflow pipe lengths 22 that cause the fluidised bed inside the risertubes to have a higher porosity than the bed inside the return tube. Thefluidised granular mass in the riser tubes will first reach the upperchamber 16 and overflow into the return tube 21 through which thegranular mass and the fluid medium will start to flow down. In the lowerchamber 17 between distribution plates 24 and 42 the downward flowthrough the return tube 21 and the upward fluid medium flow from theinlet 11 will be mixed, and finally a balanced state occurs where thevolume of fluid medium leaving the heat exchanger via outlet 12 equalsthe volume that enters the heat exchanger through inlet 11 and thegranular mass is circulating through the riser tubes 18 and the returntube 21. During the circulation process the riser tubes 18 are lessheavily charged with granular mass than the return tube 21. To suppressthe adverse effect on the average logarithmic temperature difference dueto the fluid medium flow through the return tube, circulation of thefluid medium through the return tube should be minimal. An increase ofthe pressure difference between the upper and lower chambers contributesto reducing the fluid medium flow through the return tube. The increaseof the pressure difference is achieved by fitting several distributionplates at a level above the return tube outlet into the lower chamber,preferably so that the apertures in the various distribution plates ofthe multiple design are not vertically in line. FIG. 3 shows themultiple distribution plate design 45,46,47 in the lower chamber 17above the return tube 21 outlet.

FIGS. 4 and 5 illustrate the operation and design of the device forcleaning the lateral bores in the inflow pipe elements of the risertubes. FIG. 4 shows the heat exchanger 10 as it is shown in FIG. 1 undernormal operating conditions, the riser tubes 18 functioning normally.From FIG. 4 is appears that the upper section of the lower chamber 17 isconnected, by a pipe 51 provided with a shutoff valve 52, to a dischargeline 53 that carries off the flowing fluid medium from the outlet 12.During normal operation the shutoff valve 52 is closed. The lowerchamber 17 contains a granular mass volume in fluidised condition up toa level just below the lateral bores 23 in the inflow pipe lengths 22.Fluidised granular mass is further present in the riser tubes 18 and inthe upper chamber 16. In the riser tubes 18 the granular mass and thefluid medium move upward and downward in the return tube 21. If alateral bore in an inflow pipe length of a riser tube clogs up theupward flow inside that riser tube may suddenly change into a downwardflow. To avoid this undesirable situation it should be possible toremove any dirt deposits on the outer surface of the inflow pipeelement. This is achieved by partly opening the shutoff valve 52. Thiswill cause part of the fluid medium to flow to the discharge line 53 viathe lower chamber 17 and the partly opened shutoff valve 52. Thefluidised granular mass in the riser tubes 18 will sag to a level e.g.half the height of the riser tubes, and the upper chamber 16 will notcontain any more granular mass at all. Under such conditions the returntube 21 will function as a partly filled riser tube. In the lowerchamber 17 the volume of fluidised granular mass wll be growingsubstantially, which will cause the granular mass to extend beyond thelateral bores 23 while yet keeping an adequate distance between the massand the connection of pipe 51 to the lower chamber in order to preventany granular mass from being carried off with the fluid medium. Theabrasive action of the fluidised granular mass on the lateral bores inthe inflow pipe lengths will remove the fouling deposits and carry themoff via the pipe 51.

FIG. 5 shows the position of the fluidised granular mass inside thesystem, with the shutoff valve 52 in the partly open position. Itappears that the operating condition, which varies in that the shutoffvalve 52 is partly open so that the lateral bores are being cleaned, hasto be maintained for only a few dozen seconds, which is not generallyconsidered objectionable. For inspection of the level of the fluidisedgranular mass in the lower chamber 17 during cleaning of the lateralbores in the inflow pipe elements, simple pressure differencemeasurement will be sufficient. When the shutoff valve 52 closes againthe system will return to the original operating condition.

As already said, a disadvantage of an apparatus of the type described atthe outset is that sometimes the circulation within the system of thefluid medium and of the granular mass produces an excessively adverseeffect on the average logarithmic temperature difference across the heatexchanger. The main source of the problem is fluid medium circulation.

FIG. 6 shows an embodiment of a suitable solution for this problem thatconsists of providing a bypass 61 between the inlet 11 which opens intothe lower chamber 17 for the fluid medium taking part in the process anda position 62 in the upper chamber 16 at which the bypass 61 opens atthe level of the inlet 63 of the return tube 21.

The bypass is provided with an adjustable valve 64.

At start-up of the heat exchanger it is recommended to keep the shutoffvalve 64 in the bypass 61 closed. When the system is in a stableoperating condition the shutoff valve 64 can be gradually opened topartly or fully open depending on the process requirements. A fluidmedium flow with a temperature equal to the entry temperature of thefluid medium in the lower chamber 17 is now present in the immediatearea of the inlet opening 63 of the return tube. As the result theaverage temperature inside the return tube will be nearer to the entrytemperature of the fluid medium in the lower chamber than it would be inthe absence of the bypass, which produces the beneficial effect referredto earlier. The bypass can also be located inside the heat exchanger.

Another solution for the problem that is solved by the embodiment ofFIG. 6 is according to the invention achieved by locating a throttledevice in front of the return tube inlet that gives precedence topassage of the fluid medium through the return tube over passage of thegranular mass. FIGS. 7 and 8 show an embodiment of this other solution.In FIG. 7 the throttle device 71 is a pipe that is provided as acontinuation of the return tube 21 at a distance h from the inlet 63 ofthe return tube in the upper chamber 16. In principle, the throttledevice 71 can consist of a straight pipe length the top end of whichextends far enough into the upper chamber 16 to prevent granules fromdropping through the straight pipe into the return tube 21 during allnormal operating conditions. Experimental results have shown that thedistance h affects the rate of circulation of the granular mass and thefluid medium flow through the return tube. For correct adjustment of thecirculation rate the distance h must satisfy the condition:

    0.1×Dv<h<2.0×Dv

where Dv is the inner diameter of the return tube above which thethrottle device is positioned.

By slightly changing the shape of the throttle device, the device willbe capable of performing yet another function. If insufficient care isexercised at start-up of the heat exchanger, the return tube may, asmentioned earlier, start to operate as a reiser tube carrying very largevolumes of fluid medium and granular mass from the lower chamber 17 tothe upper chamber 16. As the result the throttle device of the straightpipe type can blow the granules very high into the upper chamber andgranules may be lost via outlet 12 for the fluid medium. This is anundesirable situation. By selecting a suitable inner diameter of thethrottle device of the straight pipe type, and by bending the pipe atthe top from the vertical plane into the horizontal plane and thenbending it slightly down and again over an angle of about 90 degrees thegranules and the fluid medium that are brought at high velocity out ofthe return tube 21 are trapped in the bent pipe and are further guidedso that the granules remain in the upper chamber. FIG. 8 shows a topview of the design as a cross-section of the upper chamber illustratinghow the pipe to be used as a throttle device must be bent. The innerdiameter of the throttle device of the bent pipe type must exceed theinner diameter of the return tube to prevent granules from being lost.

A next aspect of the invention relates to apparatus for carrying outphysical and/or chemical processes, in particular a heat exchanger ofthe continuous type, consisting of a bundle of parallel vertical risertubes, an upper chamber, a lower chamber, an upper pipe plate and alower pipe plate for open connection of the pipe bundle to the upper andlower chamber respectively, all of which are provided for throughflow ofa first fluid medium, and an upper distribution chamber and a lowercollecting chamber for throughflow of a second fluid medium, the bottomof the upper distribution chamber closing around each riser tube leavinga gap between the bottom of the upper distribution chamber and the risertube wall, the top of the lower collecting chamber being open.

In this apparatus, the second fluid medium is a fluid that flows down asa film along the vertical riser tubes in counterflow with the firstfluid medium that rises inside the riser tubes. For optimal cleaning ofthe outer surface of the riser tubes or the outer surface of other pipesof the same bundle all tubes and pipes are fitted on the outside with ascraping device mounted in a supporting assembly that can be drivenmechanically. FIG. 9 shows an embodiment of the scraping device, i.e. atop view perpendicular to the pipes. In a frame consisting mainly of twoyokes 201,202 and two crossbars 203,204, spacers 205 are used to mountstrips 206,207 that fit closely around the riser tubes 118. The frame isdisplaced in the longitudinal direction along the riser tubes by meansof a spindle 208 that engages in a threaded hole 209 in the yoke 201.The yoke 202 is guided by a rod 210 that passes through a hole 211 inthe yoke. This is illustrated in FIG. 10 which shows a cross section ofthe scraping device in FIG. 9 along the line X--X shown in that figure.To allow mounting of the scraping device in the heat exchanger the stripwidth must not exceed the free gap between the pipe rows in the bundle.

In a system of the last-described type, particularly the gaps in thebottom of the upper distribution chamber that allow the fluid to flowfrom the upper distribution chamber to the film around the riser tubescan get clogged. To reduce the risk of these gaps clogging up, inaccordance with the invention at least one distribution shell isarranged in the upper distribution chamber above the base; the shell isprovided with apertures for distribution of the fluid over the gaps inthe base. FIG. 11 shows apparatus 100 of the type described, having anupper chamber 116, a lower chamber 117 and a pipe bundle 118 forthroughflow of the first fluid medium, and an upper distribution chamber101 and a lower collecting chamber 102 for throughflow of a second fluidmedium, wherein the upper distribution chamber 101 closes around eachriser tube 118 so as to leave a gap 103 in the bottom 105 of the upperdistribution chamber of the riser tube and wherein the top 106 of thelower collecting chamber 102 is open.

The upper distribution chamber is provided with a distribution plate 131with apertures 130 centred between the riser tubes 118 which extendthrough the upper distribution chamber. The presence of the distributionplate 131 allows gaps 103 of greater width than is otherwise possible.FIG. 12 which shows a top view of a section of the distribution plate131, illustrates the centred positions of the apertures 130 between theriser tubes 118. For the sake of clarity, the scale ratios do notcorrespond with an actual embodiment.

As to the operation of the system, the fluid flowing down through theapertures 130 splashes outwardly on the bottom 105 and so reaches theouter walls of the riser tubes and runs down them as a film. Thus thegap 103 does not need to contribute essentially to the formation of thefilm. To ensure that the fluid jet will splash outwardly through anaperture 130 a suitable distance must be maintained between thedistribution plate 131 and the bottom 105.

It is remarked that, whereas the distribution plate referred to abovehas been provided with apertures, these apertures may also be fittedwith tuyeres, or spray nozzles, that cause lateral outflow from anaperture.

To ensure stable operation of the various systems described above, thepressure drop in the fluid medium flow over the distribution plate intothe lower chamber directly below the riser tubes must be in excess ofone-tenth of the pressure drop due to the total weight of the granularmass, as defined in the article quoted earlier and is illustrated inDutch patent application No. 7703939.

What is claimed is:
 1. Apparatus for carrying out physical and/orchemical processes, in particular a heat exchanger of the continuoustype, comprising a bundle of parallel vertical riser tubes, an upperchamber, a lower chamber, an upper pipe plate and a lower pipe plate foropen connection of the pipe bundle to the upper and lower chambersrespectively, a granular mass that can be kept in a fluidised conditionat least in the riser tubes by a fluid medium that flows duringoperation upwardly through the lower chamber, the riser tubes and theupper chamber, a distribution plate for the granular mass in the lowerchamber, and at least one return tube with an outlet below thedistribution plate for return of an overflow of granules above the upperpipe plate from the upper chamber to the lower chamber, wherein eachriser tube is provided with an inflow element extending into the lowerchamber from the lower pipe plate to a level above the distributionplate through which the return tube or return tubes projects or project,and the lower chamber is provided with a device that prevents thegranules from reaching the lower chamber inlet for the fluid medium atstandstill characterized in that a constriction (32) is provided in theends of the riser tubes (18) that open into the upper chamber (16). 2.Apparatus according to claim 1, characterised by an upper pipe plate(19) with apertures (32) for the riser tubes (18) having smallercross-section than the riser tubes themselves.